The present invention relates to lubricating oil compositions which provide high performance standards particularly in natural gas engines.
There is continuous need for improving the performance characteristics of engines, in particular natural gas engines, including stationary gas engines and engines consuming compressed natural gas, and the lubricating oils used therein. Stationary gas engines are typically large, heavy duty, stationary engines designed to run on natural gas and other like fuels. Trends in such engines include the development of smaller four-cycle, lean burning engines, for which high performance lubricants are important.
Acceptable performance in natural gas engines requires that the lubricant maintain its good qualities in spite of the severe conditions under which the engines may operate. Various forms of deterioration of the lubricant may result from contact with acidic or corrosive gaseous products of combustion. Despite such challenges, the lubricant should exhibit low degrees of oxidation and nitration over time. Likewise, it is important that the development of acidity in the lubricant be minimized. Acidity is typically measured and reported in terms of Total Acid Number (“TAN”), ASTM D664A. In order to mitigate acidity formation, many lubricant formulations include basic compounds such as overbased detergents, which provide basicity or base reserve to the lubricant, typically measured and reported in terms of Total Base Number (“TBN”), ASTM D2896.
Lubricants containing a variety of detergents are known. For example, U.S. Pat. No. 6,727,208, Wilk, Apr. 27, 2004, discloses a lubricating oil composition comprising a major amount of an oil of lubricating viscosity and an additive system comprising (in addition to other components) from about 0.1 to about 5% by weight of a detergent composition comprising at least two metal overbased compositions wherein said detergent composition consists essentially of (A-1) at least one alkali metal overbased detergent and (A-2) at least one calcium overbased detergent, in certain defined ratios.
U.S. Pat. No. 5,726,133, Blahey et al., Mar. 10, 1998 is directed to a low ash natural gas engine oil which contains an additive package including a particular combination of detergents and also containing other standard additives such as dispersants, antioxidants, antiwear agents, metal deactivators, antifoamants and pour point depressants and viscosity index improvers. The low ash natural gas engine oil exhibits reduced deposit formation and enhanced resistance to oil oxidation and nitration. The mixture of detergents comprises at least one first alkali or alkaline earth metal salt of mixture thereof of low TBN of about 250 and less and at least one second alkali or alkaline earth metal salt or mixture thereof which is more neutral than the first low TBN salt. The metal salts may be based preferably on sodium, magnesium or calcium, and may exist as phenates, sulfonates, or salicylates. More preferably, the metal salts will be calcium phenates, calcium sulphonates, calcium salicylates and mixtures thereof.
U.S. Pat. No. 6,596,672, Carrick et al., Jul. 22, 2003, discloses low ash lubricant compositions containing multiple overbased materials and multiple antioxidants, useful in lubricating stationary gas internal combustion engines. The total sulfated ash content may be about 0.1 percent to about 0.8 percent, A calcium, barium, or strontium overbased acidic material may contribute 0.01 to 0.79 percent sulfated ash, and a magnesium or sodium overbased acidic material may contribute 0.01 to 0.79 percent sulfated ash. In an example, a concentrate for a lubricating composition is prepared by combining a lubricating oil, 14% barium synthetic sulfonate of 400 TBN (oil free), 8% sodium synthetic sulfonate of 150 TBN (oil free), 6% of a succinimide dispersant, and other components.
The disclosed technology, therefore, provides a lubricant for a natural gas engine which exhibits at least one of retention of TBN, reduction of TAN formation, reduced copper corrosion, reduced oxidation, and reduced nitration, by the defined use of an overbased alkali metal detergent.